The present invention relates to a lens body tube configured to carry out a zooming operation and/or a focusing operation with a plurality of lenses being translated along the optical axis direction of the lens body tube using a cam ring.
In a lens body tube that is configured to carry out a zooming operation and/or a focusing operation with lenses, which is incorporated in the lens body tube, being translated along an optical axis direction of the lens body tube, each of the lenses is movably supported in an inner tube acting as a guide tube for translatory movement such that each of the lenses can be translated along the optical axis direction. Further, a cam ring for which a rotating operation is to be carried out is fitted around the outer circumferential surface of the inner tube, and a cam, which is formed on the inner circumferential surface of the cam ring, is engaged with a cam follower with which each of the lenses is provided. In other words, by rotating the cam ring integrally with a zoom ring or a focus ring being rotated during the zooming or focusing operation, each of the lenses engaging with the cam is translated along the optical axis direction of the lens body tube according to the shape of the cam, so that the zooming or focusing operation is carried out.
Even when the cam ring is fitted around the outer circumferential surface of the inner tube, and the cam follower of the lens is engaged with the cam formed on the inner circumferential surface of the cam ring, in order to rotate the cam ring smoothly, it is needed to secure a gap in the radial direction of the lens body tube such that the cam is engaged with the cam follower between the inner circumferential surface of the cam ring and the outer circumferential surface of the inner tube. For this reason, conventionally, there are formed bayonet pieces that are protruded inward along the radial direction at a plurality of places in the circumferential direction on one end of the cam ring, that is, on the rear end of the cam ring to be fitted around the outer circumferential surface of the inner tube in the optical axis direction. The inward top surfaces of the bayonet pieces have contact with the outer circumferential surface of the inner tube. Further, there are formed bearing ribs at a plurality of places in the circumferential direction on the outer circumferential surface of the leading end of the inner tube. The bearing ribs have contact with the inner circumferential surface of the cam ring. Thereby, the aforementioned gap is secured. In addition, the bayonet pieces are engaged with bayonet portions provided on the outer circumferential surface of the inner tube. On the leading end of the inner tube, there is formed a flange that is protruded outward along the radial direction. The flange has contact with the leading end face of the cam ring in the optical axis direction, so that the position of the cam ring is limited in the optical axis direction, and the cam ring never drops off the inner tube in the optical axis direction.
In this configuration, each of the bayonet pieces is formed to have a longer length in the circumferential direction than a required one, and is located at a predetermined position in the circumferential direction, so that engagement between the bayonet pieces and the bayonet portions can not be unfastened even though the cam ring is rotated around the optical axis by a required angle. For example, when the rotation angle of the cam ring is 90 degrees, there are arranged three bayonet pieces in the circumferential direction. Therefore, the number of bearings for supporting the cam ring on the outer circumferential surface of the inner tube is the same number of three as that of the bayonet pieces. The number of this level of the bearings is not enough to prevent the cam ring from moving in the radial direction due to vibration and/or impact applied to the lens body tube, and as a result, backlash of the cam ring is caused in the radial direction. Such backlash can be a factor that generates some errors in the moving position of the lens in the optical axis direction corresponding to a rotation angle of the cam ring, and can thereby be a problem in the case of manufacturing the lens body tube as designed.
In order to restrain such backlash in the radial direction, Japanese Unexamined Patent Publication No. HEI8-304688 discloses a configuration where an outer tube supports a member that has elastic force, the member having elastic contact with the outer circumferential surface of an inner tube. In addition, Japanese Unexamined Patent Publication No. HEI10-170796 discloses a configuration, in a lens body tube in which a moving member is supported to move relative to a fixed body tube, where anti-backlash members are arranged on the moving member.
When the technology disclosed in Japanese Unexamined Patent Publication No. HEI8-304688 or No. HEI10-170796 is applied to the aforementioned lens body tube provided with the cam ring, the cam ring is required to have an anti-backlash member attached to a part thereof. Thereby, since the number of components and the number of processes for assembling the lens body tube are increased, there is caused a problem that the total cost of the lens body tube rises. Moreover, since it is necessary to secure a space for attaching an anti-backlash member to the lens body tube, the freedom in design of the cam is limited. On the other hand, when a convex cam is formed as a cam to be formed on the inner circumferential surface of the cam ring, it is considered one of solutions for restraining the backlash that the top surface of the cam, which faces inward along the radial direction, is made contact with the outer circumferential surface of the inner tube to increase the contact area between the cam and the inner tube. However, in the case of the cam ring being formed by resin molding as a cam ring of recent years, there is often generated a small strain due to the molding. Therefore, it is hard to fit the cam ring around the outer circumferential surface of the inner tube. In addition, even though the cam ring is fitted around the outer circumferential surface, the cam ring is likely not to be smoothly rotated because of the friction between the top surface of the cam and the outer circumferential surface of the inner tube.